The present invention relates to dynamoelectric machines and, more particularly, to dynamoelectric machines having a field rotor excited by a dc power source.
A large generator, for example, employs a rotor magnetically excited by a dc power source to produce a magnetic field which is rotated within a stationary armature. The armature includes windings which produce AC electric power as the magnetic field of the rotor rotates past them.
The rotor requires a substantial source of dc exciter power to produce a magnetic field of magnitude sufficient to drive the generator to full output under rated load. Four techniques are conventionally employed to provide the dc exciter power.
In a first technique, AC power from an external source is passed through a transformer to adjust its voltage to a value compatible with later functions. In most cases, the transformer reduces the ac voltage. A rectifier and control assembly produces dc exciter power for connection to the field rotor through slip rings. In some cases, the external source of ac power may be the output power of the generator itself.
In a second technique, a dc generator is connected to the generator shaft to produce the required dc power. This dc power is connected to the field rotor through slip rings. This technique has the disadvantage that the length of the overall generator is increased. As a consequence, the building to house the generator must be made correspondingly larger. This, and the need for the dc generator, adds substantially to the cost of the plant.
In a third technique, a stationary DC source excites ac exciter windings rotating with the rotor. A rectifier assembly in the rotor produces the required dc exciter power. This technique suffers from the weight and complexity of the rectifier assembly. In addition, the high-acceleration environment in the rotor is believed to encourage low reliability.
In a fourth technique, disclosed in U.S. Pat. No. 4,477,767, the disclosure of which is incorporated herein by reference, three slots in the armature, mutually spaced apart at 120 degrees, receive exciter or P-bars (potential bars). As the magnetic field rotates to generate output power in the normal armature windings, it also generates ac exciter power in the P-bars. This exciter power is passed through a transformer to adjust its voltage before being connected to a rectifier and control assembly for the production of dc exciter power. The resulting dc exciter power is connected through slip rings to the field rotor.
This fourth technique, sold under the trademark "Generrex PPS" by the GE corporation, has found wide acceptance in the field. However, the need for a transformer adds an increment of cost which it would be desirable to avoid, if possible.
As noted above, the transformer adjusts the output voltage of the P-bars to a value consistent with the needs of the field winding. At the same time, the current is adjusted to a value which is compatible with the requirements of the field rotor and with the capacities of commercially available rectifier devices. It has been believed heretofore that the voltages and currents that can be generated by P-bars must be adjusted in a transformer before rectification.
The isolation provided by the delta-wye connection of the transformer and the series inductances contributed by the transformer in the '767 patent conventionally are believed necessary to avoid damage in response to faults in the excitation potential winding field winding in the rotor or other sources.